A large number of writing instruments which use a temporary ink storage member and which are generally referred to as an "ink-free type writing instrument" as typically represented by a fountain pen are known. The ink-free type writing instrument has a greater advantage in that the change of ink dischargeability is smaller until ink inside an ink tank is consumed fully, than those types of writing instruments in which an ink occluder is impregnated with ink to store it.
This advantage becomes a particularly great advantage for a pen for an automatic drafting machine. For, the density of a written line does not change much before and after replacement when ink is consumed fully and ink is replenished.
However, there are limitations peculiar to this type of pen for in use an automatic drafting machine.
The first limitation resides in that a fitting portion for fitting the pen to the apparatus must be formed on the writing instrument. For this reason, an exclusive adaptor is used in most cases. In other words, a screw engagement portion is disposed in an adaptor having a flange portion for fitting to the apparatus and a screw engagement portion is disposed in a writing instrument main body, too, so that the adaptor and the writing instrument main body can be engaged with each other by these screw engagement portions. Alternatively, the adaptor itself consists of front and rear members that can engage removably with each other and engagement of these front and rear members clamps and fixes the writing instrument main body.
The use of such an adaptor is not advantageous from the aspect of the following second limitation.
The second limitation is that alignment between the pen tip and the fitting portion of the pen for an automatic drafting machine for fitting the pen to the apparatus must be extremely high. If this alignment is not sufficiently high, the written line will deviate when the pen is replaced during drawing or if concentric circles are drawn in different colors, their alignment cannot be satisfied fully. In this sense, the use of the adaptor invites dimensional variance and deteriorates alignment as much as the combined number of components.
One of the means which satisfy simultaneously the first and second limitations is to make the fitting portion integral with the writing instrument main body. As a matter of fact, some of the writing instruments of the type wherein the ink occluder is impregnated with ink and stores it have a flange portion for the apparatus which is molded integrally with a shaft by injection molding.
However, integral molding of the fitting flange portion with the shaft cannot be used directly for the ink-free type writing instruments.
For, the pen for an automatic drafting machine must have excellent in vibration resistance as the third limitation. To satisfy this requirement, dimensional accuracy of the temporary ink storage member must be sufficiently high and in order to let the temporary ink storage member exhibit fully its functions, its dimensional accuracy must be sufficiently high, as well. As is known well, the temporary ink storage member plays the function of storing temporarily excess ink by capillary force in order to prevent the excess ink from being supplied to the pen tip and from causing ink leakage when an internal pressure of an ink tank rises, the function of moving the stored ink back into the ink tank more preferentially when the internal pressure of the ink tank drops and the function of sending smoothly the air into the ink tank without increasing the drop of the internal pressure when the ink is not stored. In order for these functions to be fully provided, dimensional accuracy of each portion such as a portion for storing temporarily the ink must be sufficiently high and even when each component is disposed inside the shaft, this high dimensional accuracy must be maintained sufficiently. However, vibration resistance described above is one of the factors which impede the functions of the temporary ink storage member. Therefore, dimensional accuracy must be kept particularly high. Integral molding of the flange portion with the shaft must not impede dimensional accuracy of the temporary ink storage member and if possible, integral molding is expected to contribute rather to the achievement of the various functions of the temporary ink storage member described above.
The fourth limitation is that the writing instrument must be produced often in an extremely great number of product groups. The writing thickness range from a small thickness to a large thickness and the number of colors of ink is great, too, such as black, red, blue, green, orange, yellow, brown, and so forth. Furthermore, writing surfaces are diversified such as vellum paper, film, and so forth; hence, requirements from the aspect of application are diversified. When these requirements are combined with one another, the number of kinds of products may become some dozens of kinds or more.
The fourth limitation described above brings forth in turn the fifth and sixth limitations. In other words, the fifth limitation is that the writing instruments must be produced efficiently, though the number of their kinds is great. The sixth limitation is that each kind of product must be identified easily when it is used.